Signal derotating receiver

ABSTRACT

A receiver, for example a receiver of broadcast digital terrestrial television signals modulated using COFDM (Coded Orthogonal Frequency Division Multiplexing), imposes a phase adjustment on a received signal. Phase adjustment may be effected, for example, by sample alignment of the signal, such as for cyclic prefix removal, or by shifting a window setting for a Fast Fourier Transform (FFT) processor. Before channel estimation or decoding is performed on the information stream, the information stream is derotated to compensate for the phase adjustment previously imposed on the received signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the effective filing date under 35 USC §§120 and 363 to PCT International Application. No. PCT/GB00/04001,entitled “Television Receiver”, filed Oct. 18, 2000 designating the U.S.and published under PCT Article 21(2) in English as InternationalPublication No. WO 02/05550 A1 entitled “Television Receiver,” of whichthis application is a continuation, which PCT application claimspriority to Great Britain Patent Application No. 0017132.2, filed Jul.12, 2000. This application claims priority under 35 USC 119(a) to GreatBritain Patent Application No.: 0017132.2, filed Jul. 12, 2000.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] This invention relates to a communication receiver.

[0004] 2. Description of Related Art

[0005] The European DVB-T (Digital Video Broadcasting-Terrestrial)standard for digital terrestrial television (DTT) uses Coded OrthogonalFrequency Division Multiplexing (COFDM) of transmitted signals, whichare therefore grouped into blocks and frames.

[0006] After reception in the television receiver, the signals aresampled, for example using a resampler, and are mixed down to baseband.The start of each active symbol is found, and then the active symbolsare applied to a Fast Fourier Transform (FFT) processor, andsubsequently to a channel estimator, to extract the wanted information.

[0007] It is necessary to transmit the DTT signals over transmissionpaths which are of uncertain quality. In particular, the area close tothe transmission path may include objects such as tall buildings, whichcause echoes. That is, a signal may be received at a receiver twice,once on a direct path from the transmitter, and then, after a shortdelay, as an echo. Further, there may be no direct line of sight fromthe transmitter to the receiver, in which case the receiver will onlyreceive echoes. The effect of this is that the first signal received maynot necessarily have the strongest power. There will therefore becombinations of pre-echoes arriving before the strongest signal andechoes arriving afterwards.

[0008] As is well known, this scenario can cause inter-symbolinterference (ISI) in the receiver. To reduce the effects associatedwith this problem, DVB-T COFDM signals include a cyclic prefix guardinterval for each active symbol. Specifically, a portion of the activesymbol is repeated before the next active symbol.

[0009] Once the received signal is converted down to baseband, if thereis a large echo present, a time domain correlation between samples whichare an active window length apart yield large powers in the guardinterval of the echo. These correlations can be used to correctlyposition the window when large echoes are present, although thetechnique is not as effective for smaller echoes. If the smaller echoeslag the larger ones, then correct positioning of the windowing relativeto the first large echo (or relative to the main signal if no largepre-echo is present), will result in a good solution. On the other hand,if the smaller echo is a pre-echo, this may not be the case, as thepre-echo will be introducing ISI.

[0010] One solution to this problem is to pull back the window position,calculated using the correlations in time, which can avoid ISI, butwhich rotates the signal in the frequency domain. Large rotations in thefrequency domain can adversely affect the performance of the channelestimator. Moreover, the guard interval prefix must be removed beforethe signals are further processed. The initial position of the prefixcan be found, and it is also preferable to allow correction for anychanges in position caused by subsequent variations in sampling rate.Again such corrections have the effect of rotating the signal in thefrequency domain.

SUMMARY OF THE INVENTION

[0011] There are many possible reasons for wanting to rotate a receivedsignal, either forwards or backwards, in the frequency domain. However,such rotations can have an adverse effect on channel estimation.

[0012] According to a first aspect of the invention, there is provided areceiver circuit which includes a derotator circuit, that is a circuitwhich can apply a rotation that is equal and opposite to that previouslyapplied, before a signal is applied to a channel estimator.

[0013] According to a second aspect of the invention, there is provideda method of processing received signals, that includes applying arotation which is equal and opposite to that previously applied, beforethe signal is applied to a channel estimator.

[0014] Thus, the rotation that is applied can compensate for thatpreviously applied, thereby improving channel estimation, and ultimatelyimproving signal reception.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015]FIG. 1 is a simplified block schematic diagram of a receivercircuit in accordance with the invention.

[0016]FIG. 2 is an illustration of the operation of the derotator shownin the receiver circuit of FIG. 1.

DETAILED DESCRIPTION

[0017]FIG. 1 shows simplified block diagram of a receiver circuit orsystem in accordance with the present invention. It will be appreciatedthat many of the receiver functions can be carried out in a differentorder from that illustrated in FIG. 1 and as described below, and thatFIG. 1 is exemplary only.

[0018] Typically, in the exemplary case of a digital terrestrialtelevision signal receiver, for example receiving signals using theDVB-T standard with Coded Orthogonal Frequency Division Multiplexing,the receiver includes an antenna (not shown) and a tuner (not shown) forreceiving signals and downconverting the received signals to anintermediate frequency.

[0019] As shown in FIG. 1, the receiver further includes a further mixerstage 10, for downconverting to baseband, and a resampler 12, forobtaining digital samples of in-phase (1) and quadrature (Q) componentsof the signal.

[0020] The sampler is controllable in the sense that its samplingposition can be adjusted.

[0021] Output signals from the resampler 12 are supplied to a processingdevice 14 that removes the cyclic components preceding each activesymbol. In order to be able to do this accurately, the sampling positionof the resampler 12 must be controlled such that the assumed position ofthe start of each symbol accurately coincides with the actual positionin the received signal. This control of the sampling position isachieved by adjusting the phase of the resampler 12 under control of aresampler controller 16. Such adjustments of the phase, in effect,rotate the signal in the phase plane.

[0022] An algorithm to track the resampler displacement offset should ingeneral not have large corrections in any particular symbol. However, itmay be advantageous for it to be able to do so.

[0023] The baseband I- and Q-data signals are supplied to a Fast FourierTransform (FFT) processor 18. However, in order to avoid any problems ofinter-symbol interference (ISF) which may be caused by pre-echo signals,that is, attenuated versions of the main signal which arrive at thereceiver before the main signal does, the FFT window may be pulled backin time. Again, this has the effect of rotating the spectrum of the mainsignal.

[0024] After processing is performed by the Fast Fourier Transformprocessor 18, the data signals are supplied to a derotator block 20.

[0025] The operation of the derotator 20 is now described with referenceto FIG. 2. FIG. 2 shows the values of the I- and Q-samples at oneparticular illustrative moment in time. Ignoring the effect of therotation of the signal introduced by the resampler position correctionalgorithm and the Fast Fourier Transform processor window positionpullback, the sample values would be at the position marked PI in FIG.2.

[0026] However, the resampler position correction algorithm has alteredthe position of the signal by SP1 samples, and the Fast FourierTransform processor window position has been pulled back by a furtherSP2 samples, which have introduced a rotation which means that,thereafter, the sample values are at the position marked P2 (as shown inFIG. 2).

[0027] Each sample change in the window position produces a phase rampacross the frequency spectrum from 0 on the DC bin to 360°, or 2πradians, on the final bin of the FFT processor. Therefore, if the FastFourier Transform size is N (which may, for example, be 2048 samples),and n is the bin offset, rotation by a number of samples SP, whereSP=SP1+SP2, produces a rotation of 0 radians, where:

θ=2πn(SP/N)

[0028] The derotator 20 therefore detects the amount by which the FastFourier Transform processor window position has been pulled back, thatis, SP2 samples. The derotator 20 also detects the size of thecorrection applied to the resampler position in each symbol, and hencethe cumulative correction, that is, SP1 samples. The derotator 20 thenforms the sum SP of SP1 and SP2, and calculates the total appliedrotation θ, as described above.

[0029] As is well known, a rotation of a complex value can be achievedby complex multiplication, and, in this case, an equal and oppositerotation is applied to compensate for that previously applied.

[0030] Specifically, the corrected sample position S2, having I- andQ-values IS2 and QS2, where:

S 2=IS2+jQS2,

[0031] is obtained from the input sample position S1, having I- andQ-values IS1 and QS1, where:

S 1=IS1+jQS1,

[0032] by means of the complex multiplication:

IS2+jQS2=(IS1+jQS1)e ^(−jθ).

[0033] Referring again to FIG. 1, the output data signal output by thederotator 20 is then input to a channel estimator 22 including anequalizer, demultiplexer and deinterleaver 24 and decoder 26, whichrecover the originally transmitted bitstream, in a generallyconventional way.

[0034] The channel equalizer relies upon the channel being steady formultiple symbols. If a large resampler displacement offset is added,then the large phase ramp introduced will introduce an apparent rapidchange in the channel and thus degrade the channel equalizerperformance. The performance of the channel estimator can be optimizedby removal in the derotator 20 of any previously applied rotation, thusimproving the performance of the device. Specifically, the derotator cancompensate for the introduced phase ramps, and therefore rapid movementsin window position are possible, without degrading performance.

[0035] For example, in a mobile environment, the window position mayadvantageously be rotated either forwards or backwards. Although theinvention has been described above in terms of a forwards rotation beingcompensated by a backwards derotation, it will be appreciated that theinvention is equally applicable to compensating a backwards rotation ofthe window, by means of a forward rotation.

[0036] The receiver system has been described herein with all of thecomponents on a single device, such as a large scale integrated circuit.However, it will be appreciated that the different functions may beachieved in different devices, and in different ways from thosedescribed.

What is claimed is:
 1. A receiver comprising: a) a phase adjustmentcircuit changing a phase of received signals; b) a transformationprocessor deriving frequency domain representations of phase-adjustedsignals from the phase adjustment circuit; c) a derotator applying arotation to an output of the transformation processor to counteract anyadjustment applied by the phase adjustment circuit; and d) a channelestimator estimating channel characteristics based upon an output of thederotator.
 2. The receiver of claim 1, wherein the transformationprocessor operates with a processor window having a window position, andwherein the phase adjustment circuit is configured to adjust phase bychanging the window position of the transformation processor.
 3. Thereceiver of claim 2, wherein the phase adjustment circuit furtheradjusts phase by changing a timing of received signal sampling to alignsample timing to a determined feature of the received signal.
 4. Thereceiver of claim 3, further comprising an input section configured toprovide baseband signals to a resampler that provides I and Q componentsof the received signal for processing by the transformation processor,wherein the phase adjustment circuit controls sample timing of theresampler and window position for the transformation processor.
 5. Thereceiver of claim 4, further comprising a cyclic component removalprocessor configured to receive the I and Q components from theresampler, and to provide to the transformation processor a portion ofthe I and Q components that does not include cyclic components.
 6. Thereceiver of claim 1, wherein the phase adjustment circuit introducesphase changes by changing a timing of received signal sampling to alignsample timing to a determined feature of the received signal.
 7. Thereceiver of claim 6 wherein the phase adjustment circuit includes aresampler for forming digital samples of the received signal, and isconfigured to adjust a sample position of the resampler.
 8. The receiverof claim 1, wherein the phase adjustment circuit introduces phasechanges by changing a window position of timing of received signalsampling to align sample timing to a determined feature of the receivedsignal.
 9. The receiver of claim 1, wherein the receiver apparatus isconfigured to receive COFDM television signals.
 10. A method ofprocessing a received signal, comprising: a) shifting a phase of thereceived signal to generate a phase shifted signal; b) Fouriertransforming information from the phase shifted signal to generate phaseshifted transformed signal information; c) derotating the phase shiftedtransformed signal information as compensation for phase shifting instep a) to generate derotated signal information; and d) applying thederotated signal information to a channel estimator.
 11. The method ofclaim 10, wherein the act of a) shifting a phase of the received signalcomprises adjusting sample timing of the received signal to effectalignment of samples to a signal feature.
 12. The method of claim 10,wherein the act a) of shifting a phase of the received signal compriseschanging a window position of a Fast Fourier Transform (“FFT”)processor.
 13. The method of claim 12, wherein the act a) of shifting aphase of the received signal further comprises aligning sample timing ofa resampler to cause the phase shifted signal to be aligned for cyclicinformation removal.
 14. The method of claim 13, further comprising astep e) removing cyclic information from the aligned phase shiftedsignal to generate information from the phase shifted signal for theFourier transformation of step b).
 15. A communication signal receivingsystem, comprising: a) a sampling block configured to derive samples ofa received signal; b) a phase adjustment block configured to adjust aphase of the received signal by changing sample timing of the samplingblock to align the sample timing; c) a cyclic section removal blockconfigured to derive stripped signal information for further processingby removing cyclic sections from the derived samples; and d) a derotatorblock configured to compensate for phase adjustment imposed by the phaseadjustment block by rotating phasing of the stripped signal informationto provide derotated signal information.
 16. The system of claim 15,further comprising a channel estimation block configured to derivechannel estimates from the derotated signal information.
 17. The systemof claim 15, further comprising a Fast Fourier Transform (“FFT”)processor for providing the stripped signal information in frequencydomain form to the derotator block.
 18. The system of claim 17, whereinthe phase adjustment block is configured to further adjust the phase ofthe received signal by changing a window position of the FFT processor.19. The system of claim 18, further comprising a channel estimationblock configured to derive channel estimates from the derotated signalinformation.
 20. The system of claim 19, further comprising anequalizer, a demultiplexer, and a decoder.
 21. The system of claim 15,wherein the sampling block is configured to derive I and Q componentsamples.
 22. The system of claim 15, wherein the phase adjustment blockis configured to align the sample timing with a feature of the receivedsignal.
 23. A method of processing a received signal, comprising: a)shifting a phase of the received signal with a controller to alignreceived signal samples; b) removing cyclic information from thereceived signal samples to produce cyclically stripped signalinformation; c) derotating the cyclically stripped signal information tocompensate for any phase shifting in step a); and d) decoding signalinformation resulting from step c).
 24. The method of claim 23, whereinthe step d) comprises performing channel estimation.
 25. The method ofclaim 24, wherein the step a) comprises adjusting sample timing of aresampler to align the samples with an actual start position of thereceived signal.
 26. The method of claim 23, further comprising a stepe) performing a Fourier transformation of the stripped signalinformation.
 27. The method of claim 26, wherein the step c) comprisesperforming a complex multiplication.
 28. The method of claim 27, whereinthe step e) comprises employing a Fast Fourier Transform (“FFT”)processor, and the step a) further comprises adjusting a window positionof the FFT processor.
 29. The method of claim 23, further comprisingmixing the received signal down to baseband before performing any of thesteps a), b), c) or d).
 30. A receiver comprising: a) means foradjusting a phase of received signals to produce phase adjusted signals;b) means for deriving frequency domain (“FD”) representations of thephase-adjusted signals; c) means for derotating the FD representationsof the phase adjusted signals to counteract phase adjustment applied tothe received signals by means (a) to produce derotated signals; and d)means for estimating channel characteristics based upon the derotatedsignals.
 31. The receiver of claim 30, wherein the means for deriving FDrepresentations of the phase-adjusted signals comprises means forshifting a Fourier Transformation window position to adjust phase of thephase adjusted signals.
 32. The receiver of claim 30, wherein the meansfor adjusting a phase of received signals comprises means for changing atiming of received signal sampling to align sample timing to adetermined feature of the received signal.